1. Field of the Invention
The invention relates generally to optical elements that are fabricated utilizing thin films and to optical instruments that include such elements. More particularly, the invention relates to optical elements that are fabricated utilizing a free standing (self supporting polycrystalline continuous thin film of diamond combined with a non-hydrogenated amorphous diamond like carbon (DLC) film having a high percentage of sp.sup.3 bonding.
Optical elements fabricated from the aforementioned combination of materials may be designed to have optically smooth surfaces, have a wide optical transmission range and exhibit exceptional durability characteristics. In turn, optical instruments that are designed to include such elements may realize derivative benefits such as improved operating performance and lower maintenance requirements.
2. Description of the Related Art
A variety of techniques are well known for fabricating electronic and opto-electronic elements on different types of substrates. Such techniques may, for example, involve photolithographically etching metal film, where specially designed masks are used to obtain a desired pattern on the film.
Techniques for creating a specific type of thin film, namely a polycrystalline continuous thin film of diamond, are also well known. For example, a chemical vapor deposition (CVD) technique for depositing diamond films on a substrate is taught by Hacker et al in U.S. Pat. No. 4,948,629.
The known deposition techniques have been refined so that it is now possible to deposit polycrystalline continuous thin films of diamond on specially treated substrates that give a high nucleation density. The resulting films have small grains and smooth surfaces which are particularly well suited for optical applications, such as passing infrared (IR) light. Such teachings are, for example, set forth in an article entitled "Diamond Membrane For X-ray Lithography", by Guarnieri, et al, published in volume 1326 of Diamond Optics III (1990).
Still further examples of improved techniques for the deposition of diamond film, and in particular technique for making films that are well suited for the fabrication of optical elements, are those processes that use electrophoretic deposition of diamond powder onto the substrate. These techniques yield a denser and more uniform deposit then drip or drop methods, and are described, for example, in an article entitled "Electrophoretic Deposition of Nucleating Layers for Polycrystalline Diamond Film Deposition", by Cuomo, et al, appearing in the IBM Technical Disclosure Bulletin, Volume 33, Number 10A, published in March, 1991.
It is now well settled that diamond thin films are ideally suited for a variety of applications, for example, the fabrication of x-ray masks as referred to in the aforementioned Guarnieri, et al article, because of diamond's well known desirable properties of mechanical strength, optical and x-ray transparency, chemical inertness, flatness, rigidity and radiation hardness.
By selecting the proper deposition conditions from those techniques presently available, those skilled in the art are able to obtain films with uniform tension, that combined with both the mechanical and chemical durability of the diamond film, allows the central portion of the film to be etched out resulting in a flat, thin self-supporting diamond membrane framed over a relatively large area. These films, as will be demonstrated hereinafter with reference to the Detailed Description of the invention, may serve as ideal achromatic substrates for optical elements, particularly those that are designed to pass IR light since the IR transmission property of diamond is better than any known material.
The optical properties of self-supporting diamond thin films have been demonstrated to show the expected excellent transmission over a broad range from far infrared into the visible where scattering by the surface roughness begins to cut down on the transmission. Specific reference may be made to an article by Bi et al, entitled "Optical Properties of Chemical-Vapor-Deposited Diamond Films", appearing in the Journal of Material Research, volume 5, published in April 1990to support these results. By using the techniques described in the aforementioned Guarnieri et al reference to produce films that are smoother than those described by Bi et al, high transmission capability can extend well into the visible range.
Another type of carbon film, well known in the prior art, which has been used for a variety of purposes, including presenting a smooth surface having excellent etching resistance and which can be used as a protective or electrically insulating member for electronic devices, are diamond like carbon (DLC) films such as the one described by Ikoma et al in U.S. Pat. No. 4,935,303. Samples of the Ikoma et al DLC film have a density greater than 1.8g/cm.sup.3, up to approximately 2.7 g/cm.sup.3 (as compared with actual diamond having a density of 3.5 g/cm.sup.3), and may be produced using a magnetized microwave plasma CVD method where a magnetic field is applied. The hydrogen content of Ikoma et al's DLC is approximately 20%.
Although teaching how to make a hydrogenated DLC film and considering its protective and insulating features for the fabrication of electronic devices, Ikoma et al's teachings did not extend to DLC films that would enhance the durability of optical devices per se. In fact, DLC films that are virtually transparent and would not significantly degrade the performance of optical elements did not exist.
The DLC films having the significantly increased density characteristics and bonding characteristics referred to hereinabove, are described by Angus et al in Paper Number 8.01, published at the International Conference On New Diamond Science and Technology, in September 1990, and by Cuomo et al in a paper entitled "The Effects of Substrate Conditions On the Microstructural Evolution Of Thin Diamond-Like Films", presented at the Fall Meeting of the Materials Research Society, in Boston, Mass., in December 1990.
In these articles, an evolution towards denser films is described as taking place upon increasing beam energy, reducing substrate temperature and increasing substrate thermal conductivity. The higher density film described contain a higher fraction (approximately 80%) of tetrahedrally coordinated (sp.sup.3 -bonded) carbon; have a density that can approach 3.3 g/cm.sup.3 (i.e. a density that closely approximates the density of actual diamond); and have a hydrogen content of 5% and 1% when formed using sputter and laser deposition techniques, respectively.
Once again, the newer DLC films, referred to herein as non-hydrogenated amorphous diamond like carbon films, are not to be confused with hydrogenated carbon films (such as the film taught by Ikoma et al in the aforementioned patent), which contain more than 20% hydrogen, are substantially less dense than the non-hydrogenated films (thereby affecting the transmission of closely approximate the density of diamond); but which otherwise exhibit "diamond like" properties.
With the advent of the non-hydrogenated amorphous diamond like carbon film having sp.sup.3 fractions approaching diamond, and the refinement of techniques for depositing, high quality diamond films onto substrates that can be etched out to form flat, thin, self-supporting diamond membranes framed over relatively large areas (referred to hereinabove), it would be desirable to provide achromatic substrates and packages formed of the aforementioned materials, for use in fabricating optical elements. Such a combination (the free standing polycrystalline diamond thin film combined with the non-hydrogenated DLC film), would facilitate the fabrication of windows and optical elements that are durable and which have optically smooth surfaces. Such surfaces have heretofore been difficult to achieve without specially treating the substrate surface, etc.(as taught in the aforementioned Guarnieri et al reference), because of the relatively rough surface area presented by the inherently faceted face of diamond film.
Furthermore, it would be desirable to fabricate optical elements that are specifically designed to pass IR light, using diamond film as a substrate and the non-hydrogenated DLC film as a protective coating, since the IR transmission property of diamond, as indicated hereinbefore is better than any known material and would not be appreciably degraded by the DLC coating.
In addition to the desirability of utilizing the aforementioned materials in the fabrication of optical elements in general, and in the fabrication of IR elements in particular, it would be desirable to modify the design of well known optical instruments to incorporate such elements and realize derivative benefits, such as improved operating performance and lower maintenance requirements.
For example, beam splitters are especially important in infrared (IR) Fourier Transform Spectrometers, also known as FTIRs. However, the beam splitters presently used in spectrometers are restricted in the spectral range they can cover partly due to the limited window of transmittance of the substrate material used to support the beam splitting film. Thus, utilizing state of art beam splitters in such applications as FTIRs, requires the use of two, three or more beam splitters in sequence to cover a broad spectral range.
The same limitation applies for beam polarizers. There are far infrared polarizers made from free standing grids wound from very thin wire. Though they are free of substrates the fragility of self-supporting thin wires limit the linear density of the wires and hence are effective only in the long wavelength region, i.e. wavelengths greater than 25 micron.
Modern holographic lithography make possible the preparation of much finer grid dimensions, on the order of 250 nm, which extends the cutoff range of wire grid polarizers beyond the visible. These however require substrates which in most cases add the transmittance range limitations mentioned hereinabove.
Accordingly, it would be desirable, as a further aspect of the invention, to be able to use the aforementioned combination of materials (diamond thin film substrate and non-hydrogenated DLC film), to fabricate optical elements that can be incorporated into redesigned optical instruments, such as Fourier Transform Infrared (FTIR) spectrometers. In particular, it would be desirable to utilize the new combination of materials to fabricate wire grid polarizers and polarizing beam splitters for use in FTIRs and other optical instruments.
A specific desirable application for the invention would be in an FTIR designed around a polarization Michaelson interferometer (PMI), as described in Infrared Physics, by Martin and Puplett (1969), which would enable the PMI to cover the entire range from far infrared into the visible portion of the spectrum without the need of exchanging beam splitters and polarizers as is presently necessary when the PMI is fabricated using state of the art IR optical components.